This invention relates to improvements in an unsaturated type optical isolator for applying an undersaturated magnetic field to magneto-optical elements, and more specifically to an optical fiber terminal fitted with a small optical isolator for such applications as semiconductor laser modules and optical amplifiers.
Optical isolators are used in optical amplifiers, semiconductor laser modules, and the like. For example, a semiconductor laser module, as diagrammatically shown in FIG. 2, comprises a laser diode 1, primary lens 2, optical isolator 3 (which consists of a polarizer 10, a magneto-optical element 11 which functions as a Faraday rotator, a polarizer 12, and a magnet 5), secondary lens 6, ferrule 7, and an optical fiber 8 supported by the ferrule. Rays of light emitted by the laser diode 1 are made parallel by the primary lens 2 and, after passage through the optical isolator 3, focused by the secondary lens 6 at one end of the optical fiber 8. Return light in the reverse direction is shut off by the optical isolator 3 so as not to reach the laser diode 1. In this conventional arrangement the optical isolator 3 is accommodated in a housing 9' and the optical fiber 8 is fixed in position by welding the ferrule 7 that holds it into a slide ring 28 and welding the ring, in turn, to a lens holder 9 of the secondary lens 6.
An unsaturated optical isolator (a magnetic garnet as a magneto-optical element to which a magnetic field short of saturation is applied), as introduced by the present inventors in Japanese Patent Application Public Disclosure No. Hei 4-31821, exhibits outstanding temperature properties. However, that isolator has the disadvantage of large size that is necessitated by the use of a sufficiently large magnet to ensure uniform magnetic field distribution, since otherwise the magnetic field makes the angle of Faraday rotation variable. In an effort to overcome the disadvantage, Patent Application No. Hei 3-354071 proposed a new construction which permits some size reduction. Briefly stated, the proposed optical isolator comprises a cylindrical magnet whose direction of magnetization is axial, a soft-magnetic sleeve in the form of a cylinder disposed around the magnet substantially in contact with it, an annular magnetic yoke located at an open end of the cylindrical magnet, away from the cylindrical soft-magnetic yoke, and a magneto-optical element disposed inside the cylindrical magnet. The adoption of a soft-magnetic outer sleeve renders the magnetic field inside the cylindrical magnet less susceptible to external magnetic fields. Also, designing the magnet so that its geometry desirably satisfies the relation, length/inside diameter=0.5 outside diameter/inside diameter+(0.03.about.0.26) ensures the uniformity of the magnetic field that is applied to the magneto-optical element located inside the magnet. The isolator thus is suited for use with an unsaturated Faraday rotator with good temperature stability. Further, the magnetic field can be adjusted to any desired set value through the adjustments of the dimensions, especially the thickness, of the magnetic yoke. These simplify the process. Contrary to the common knowledge in the art with an arrangement in which the magnet is in contact or substantially in contact with the soft-magnetic sleeve, the intensity of the center magnetic field in the bore of the magnet does not decrease appreciably because the magnetic yoke is not in contact with the soft-magnetic sleeve. Still, the optical isolator of the construction described is unable to achieve satisfactory size reduction.
In order to keep the characteristics of an unsaturated optical isolator unchanged with dispersions in the position of incoming light and also in beam diameter, it is inevitable to increase the size of the isolator. However, the isolator must nevertheless be reduced in size to overcome the foregoing problem.
The characteristics of an optical isolator must be kept constant because the point of light incidence upon the isolator and the beam diameter are both variable. For example, in a conventional arrangement shown in FIG. 2, dispersions of dimensions of the component members can cause changes in the position of light that comes into and out of the optical isolator and also in the diameter of the light beam. The optical isolator is thus required to maintain its characteristics unaffected even by slight changes in the light incidence position and in the beam diameter.